1. Field of the Invention
The present invention relates to a composite multiplexer circuit capable of multiplexing high-frequency signals in plural different frequency bands, and to a chip component, high-frequency module and radio communication apparatus using the same.
2. Description of the Related Art
Portable telephones have been spreading wider and wider in recent years, while the function of the portable telephones and the services associated therewith have been improved.
A multiband portable telephone has been proposed as a novel portable telephone. In contrast to the conventional portable telephones handling only one transmission/reception system, the multiband portable telephone is adapted to handle a plurality of transmission/reception systems. This permits a user to selectively use any one of the transmission/reception systems that is suited for a service area or usage purpose.
In another practice, a reception system having a novel frequency band (1500 MHz-band) such as of the GPS (Global Positioning System) for positional detection has been added to the portable telephones.
By way of example, a brief description is made on the configurations of a portable telephone based on Cellular (800 MHz-band)/GPS systems and of a portable telephone based on PCS (Personal Communication Services: 1900 MHz-band)/GPS systems.
In addition, a brief description is also made on the configuration of a portable telephone based on Cellular/GPS/PCS systems which is adapted to switch to any one of the three transmission/reception systems of Cellular/GPS/PCS.
First, the Cellular/GPS portable telephone is provided with an antenna multiplexer which multiplexes complex signals of two Cellular/GPS transmission/reception systems having different pass bands by means of respective multiplexer circuits each allowing the passage of a high-frequency signal in a predetermined frequency band, and which switches the Cellular transmission/reception system between its transmission system Tx and its reception system Rx. The telephone further includes a power amplifier constituting the Cellular transmission system Tx, a low noise amplifier constituting the Cellular reception system Rx, a low noise amplifier constituting a GPS reception system Rx and the like.
On the other hand, the PCS/GPS portable telephone is provided with an antenna multiplexer which multiplexes complex signals of two PCS/GPS transmission/reception systems having different pass bands by means of respective multiplexer circuits each allowing the passage of a high-frequency signal in a predetermined frequency band, and which switches the PCS transmission/reception system between its transmission system Tx and its reception system Rx. The telephone further includes a power amplifier constituting the PCS transmission system Tx, a low noise amplifier constituting the PCS reception system Rx, a low noise amplifier constituting a GPS reception system Rx and the like.
Furthermore, the Cellular/GPS/PCS portable telephone is provided with an antenna multiplexer which multiplexes complex signals of three Cellular/GPS/PCS transmission/reception systems having different pass bands by means of respective multiplexer circuits each allowing the passage of a high-frequency signal in a predetermined frequency band, and which switches the Cellular transmission/reception system between its transmission system Tx and its reception system Rx and also switches the PCS transmission/reception system between its transmission system Tx and its reception system Rx. The telephone further includes a power amplifier constituting the Cellular transmission system Tx, a low noise amplifier constituting the Cellular reception system Rx, a power amplifier constituting the PCS transmission system Tx, a low noise amplifier constituting the PCS reception system Rx, a low noise amplifier constituting a GPS reception system Rx and the like.
In the GPS reception system Rx thus multiplexed by the aforesaid composite multiplexer circuit, a SAW (Surface Acoustic Wave) filter having a sharp pass characteristic is employed for exclusively allowing the passage of a reception high-frequency signal at GPS frequencies.
In this case, it is desirable that the multiplexer circuit of the GPS reception system Rx has infinite input impedances in the other frequency bands such as of the Cellular and PCS systems as viewed from an antenna terminal of the composite multiplexer circuit. In actual fact, however, the input impedances in these frequency bands are significantly low.
FIG. 34 is an impedance chart where the center C of the circle represents a characteristic impedance of 50 Ω. A solid dot R represents an input impedance of a single SAW filter as determined at a Cellular frequency band of 800 MHz which is lower than the GPS frequency band of interest. A real part of the impedance at the point R on the coordinate circle substantially has a value of 0, whereas an imaginary part thereof has a value of about −0.5. Hence, an absolute value of the impedance is roughly a half of 50 Ω or about 25 Ω. The value of 25 Ω is so small that the composite multiplexer circuit detrimentally multiplexes the high-frequency signals although it is desired to inhibit the high-frequency signals from entering the multiplexer circuit of the GPS reception system Rx. Consequently, the high-frequency signal to flow through the Cellular multiplexer circuit is decreased in power.
The aforementioned short absolute value of the impedance may also be encountered in another higher frequency band such as the PCS frequency band of 1900 MHz.
It is therefore an object of the invention to provide a composite multiplexer circuit comprising a plurality of multiplexer circuits connected in parallel thereby attaining an ability to multiplex a plurality of frequency bands, the composite circuit capable of positively preventing each of the multiplexer circuits from encountering invading signals in the other frequency band(s) than the band thereof, and of avoiding the increase of transmission losses in the individual frequency bands.
It is another object of the invention to provide a chip component, a high-frequency module and a radio communication apparatus which employ the aforementioned composite multiplexer circuit for achieving low transmission losses and high reliability.